The present invention generally relates to mobile communication system, mobile terminals, base stations, mobile switching stations and mobile communication control method, and more particularly to a mobile communication system, a mobile terminal, a base station, a mobile switching station and a mobile communication control method which transfer information using asynchronous transfer mode (ATM) cells.
Various kinds of mobile communication systems are known which connect a mobile terminal such as a car telephone and a portable telephone to a base station via a radio line, and connect the base station and a mobile switching station via a wire line. The utilization efficiency of radio frequencies on the radio line is improved using frequency division or time division multiplexing. In such mobile communication systems, there are demands to further improve the system efficiency.
FIG. 1 is a system block diagram showing an example of a conventional mobile communication system. The mobile communication system shown in FIG. 1 includes mobile terminals (or stations) 101.sub.1 through 101.sub.k such as car telephones and portable telephones, base stations 102.sub.1 through 102.sub.m, a mobile switching station (or center) 103, a general switched network 104, a base station controller 105, and general telephones 106.
The base stations 102.sub.1 through 102.sub.m which connect to the mobile switching station, and the mobile terminals 101.sub.1 through 101.sub.k which are mobile, are connected via radio channels respectively made up of a control channel and a plurality of communication channels. The base stations 102.sub.1 through 102.sub.m cope with call information from the mobile switching station 103 and call the mobile terminals 101.sub.1 through 101.sub.k using the control channel. In addition, the mobile terminals 101.sub.1 through 101.sub.k can call using the control channel. The base stations 102.sub.1 through 102.sub.m specify the communication channels between the base stations 102.sub.1 through 102.sub.m and the mobile terminals 101.sub.1 through 101.sub.k via the control channel. Talk information is transferred between the mobile terminals 101.sub.1 through 101.sub.k and the base stations 102.sub.1 through 102.sub.m by use of the specified communication channels.
In addition, a base station control station 105 controls registration of positions where the mobile terminals 101.sub.1 through 101.sub.k are located, and when the mobile terminals 101.sub.1 through 101.sub.k are called, controls the base stations 102.sub.1 through 102.sub.m and the mobile switching station 103 by referring to a home memory (not shown) which registers the position of the mobile terminals 101.sub.1 through 101.sub.k, so as to call the mobile terminals 101.sub.1 through 101.sub.k by use of the control channel. The functions of the base station control station 105 may be provided in the mobile switching station 103.
The base station control station 105 also collects and monitors intensities of received signals during communication between the mobile terminals 101.sub.1 through 101.sub.k and the base stations 102.sub.1 through 102.sub.m. For example, if the mobile terminal 101.sub.1 moves during the communication and the received signal intensity of one base station 102.sub.1 decreases and the received signal intensity of another base station 102.sub.2 increases, the base station control station 105 sends an instruction to the mobile switching station 103 to switch from the base station 102.sub.1 having the decreased received signal intensity to the base station 102.sub.2 having the increased received signal intensity. The mobile switching station 103 switches the base stations in response to this instruction, and the switched base station 102.sub.2 instructs the mobile terminal 101.sub.1 to switch to a new communication channel. As a result, the communication channel is switched to the new communication channel, and the communication can be continued while the mobile terminal 101.sub.1 moves.
Conventionally, the radio channels between the base stations 102.sub.1 through 102.sub.m and the mobile terminals 101.sub.1 through 101.sub.k employ various multiplexing techniques in order to improve the utilization efficiency of the radio frequencies. Frequency division multiple access (FDMA), time division multiple access (TDMA), time division multiple access/time division duplex (TDMA/TDD) and code division multiple access (CDMA) are examples of such multiplexing techniques. Further, with respect to the data transfer, there is a known technique which transfers the data in packets.
According to the conventional mobile communication system, the mobile terminals 101.sub.1 through 101.sub.k communicate by occupying the communication channels specified from the base stations 102.sub.1 through 102.sub.m, and the utilization efficiency of the radio frequencies can be improved by subjecting an audio signal to a band compression coding. However, the audio signal includes a relatively large amount of silent (or unvoiced) intervals, and the communication channels are occupied even during such silent intervals.
In addition, if the mobile stations 101.sub.1 through 101.sub.k move among service areas of the base stations 102.sub.1 through 102.sub.m during communication, the base stations 102.sub.1 through 102.sub.m are switched and the communication channels are switched, thereby requiring a complex control so as not to interrupt the communication. As a result, there was a problem in that the load on the processes of the mobile switching station 103 and the base station control station 105 is extremely large.